Preamp and impedance question

@georgehifi yes, can’t find the curve for the X5.  Actually, the current price without upgrade only have 8ohm tap.  In your opinion, is 8 ohm ok, or is it worthwhile to add $500 to upgrade the transformer, which will then have a 4 ohm tap and likely will improve the sound?  

@erik_squires Currently, he will use Tango XE-20s transformer.  I hope it is a good quality transformer.  But if I want to have a 4 ohm speaker output, I need to add $500 to upgrade to Tango FC-30-3.5s.  So, for the additional $509, I get a 4ohm output and likely better sound, but don’t know how significant it is.  The ultimate question is, should I do this upgrade .... I think I don’t need 4 ohm.  My current speaker has a power subwoofer.  So, it seems the dip in impedance at low frequency is not a concern for now.  but I may change my speaker, or I may sell the amp.  To ask the question a different way, when you buy an amp, is it important to you for the amp to output 4 ohm?

@georgehifi 
thats good news that I can just add the volume pots. =)

my current speaker is a Spatial Audio X5.  It has a power subwoofer.  When I say my speaker dips to 5 ohm at 30Hz, it is for my old Zu Omen Def.  And that is based on my memory, so, number may not be exact.  I just use that as an example, easier to give some numbers.

and I remember when I use my Zu, I feel the 4 ohm tap gives more bass.  But it can also be a psychological impression, nothing scientifically measured.

@gs5556 
So, my understanding is that, as long as I listen in low to medium volume, paying extra to get a 4 ohm tap will not have audible improvement over the 8 ohm tap, am I correct?

@roberttdid 
You bring up a good point.  I know the X5 use a 12” Eminence driver.  I look up their web site, and found the model and curve below.  It seems it dips down to 7 ohm around 200Hz.  So, only 1ohm lower than the nominal 8ohm.  In this case, I guess having only an 8ohm tap is ok?

And for the power subwoofer on the speaker, I guess the output impedance of the amp is irrelevant, right?

https://www.eminence.com/speakers/speaker-detail/?model=KL3012CX_8

Thank you very much for all the inputs.  I need some time to digest the explanations.  I don’t understand all of them, but I feel this thread has the answers that I have been searching on the internet.  For now, I am good!  I will add only the volume pots, and not upgrade the transformer.  

@georgehifi Please excuse my limited knowledge, I don’t fully understand the phase angle and stuff.  But to clarify, my X5 has one 12” Eminence driver and one 12” powered Eminence subwoofer by Hypex Fusion NCore plate amp.  

The cross over frequency is 90Hz. AMT - 1 kHz and up
12” mid range - 90Hz to 1 kHz
12” powered subwoofer - 90Hz and below
But that 90Hz to 1 kHz is exactly where the impedance dips to 7 ohm.

actually, I can do a test on my current amp which supports 4 ohm.  And I have been connecting my X5 to 4 ohm.  I did a brief test earlier and I remember there is no difference on my X5.  But I remember there is an improvement to use the 4 ohm tap on my Zu Omen Def.  So, it seems this really depends on the speaker.  Maybe the phase angle is less relevant in a open baffle speaker?  
But I may change speaker or I may sell this.  Definitely having a 4 ohm tap is more future proof, but I am not sure if I want to spend 20% more for the unknown future.  If I happened to buy a new speaker, maybe I should just buy another better matching amp .... decisions,  decisions .... 

Guys, I did more research and realized the Zu Omen Def is actually 6 ohm nominal!  No wonder I hear better sound with the 4 ohm tap!